A significant proportion of the water systems supplying potable water in the major U.S. municipalities were built between 1850 and 1950. The subsurface water mains which convey the water from its source to the consumer were made almost exclusively of ferrous metals such as steel and cast iron. With rare exceptions these pipelines were placed in service with little or no protection against internal corrosion. The rare exceptions included the early use of Portland or natural cement mortar to line the inside of certain water mains before they were placed underground. Subsequent experience has demonstrated that such cement mortar linings were by far the most effective means of permanently protecting water mains against internal corrosion.
Among the serious problems caused by internal corrosion are a reduction in carrying capacity, perhaps as high as 50 to 75 percent of the potential capacity, and rusty (red) water being carried to consumers' taps. The reduction in capacity is the result of the growth of barnacle-like nodules known as tubercules on the inside wall of the pipe. The tubercules cause turbulence which, in combination with the reduced cross-sectional area of the pipe, reduces the hydraulic carrying capacity of the water mains. Cleaning the mains will return the capacity to its original value temporarily, but it will return to the reduced value in a very short time. The rusty or red water is the result of corrosion products being carried in the water stream.
While some individual pipe lengths have been factory cement mortar lined since the mid-1800's, the general use of cement mortar lining was not firmly established until about 1945. Today nearly all cast or ductile iron pipe is cement mortar lined at the factory as a part of the manufacturing procedure.
During that period when water mains were being installed before the recognition of the importance of cement mortar lining, many miles of pipe were installed. Repeat cleaning was about the only means of maintaining water system capacity.
In the mid-1930's Albert G. Perkins developed a process for cement mortar lining pipelines in place. Several patents in the 1930's and early 1940's were granted as a result of the development of the lining process, examples being U.S. Pat. Nos. 1,988,329 and 2,168,917. As shown in both of these patents, a wet cement mortar is thrown centrifugally from a rotating distributor head out against the inner surface of the pipe. When properly applied, the mortar will adhere to the pipe surface until it hardens whereupon it forms a unitary self-supporting structure within the pipe. The Perkins patents also show rotating trowels mounted behind the distributor head to smooth the lining material before it hardens. Additionally, U.S. Pat. No. 1,988,329 shows a rotating spray head through which asphaltum curing control liquid is pressure sprayed onto the freshly trowelled lining. U.S. Pat. No. 2,168,917 shows an alternative means for applying a curing control liquid, namely a second electrically driven centrifugal distributor head. The application of the process shown in the foregoing patents was limited to pipelines which were sufficiently large in diameter to permit manual operation of the machines within a pipeline. The minimum diameter which could be lined was about 24 inches.
In the early 1950's a process was developed which permitted the lining of mains of less than 24 inches in diameter, too small for a man to enter. U.S. Pat. No. 2,704,873 was granted to K. K. Kirwan and Alfred G. Perkins, one of the applicants of the present application, on this process.
Since that time the process has been refined and additional patents have been granted, for example, U.S. Pat. Nos. 2,758,352; 3,044,136; 4,067,680; and 4,252,763. Today pipelines as small as four inches in diameter are lined in place.
A conventional water main is indicated generally at 10 in the drawings. As is well known in the art, if the water main is of a ferrous material and has not been coated prior to being placed into service, it will eventually become corroded and tuberculated. Thus, tubercules will gradually form on the interior wall of unprotected metallic water lines to reduce the hydraulic carrying capacity of a water transmission and distribution system. Such water lines can be restored to "new pipe" hydraulic carrying capacity by cleaning and cement lining. In practicing the prior art, temporary distribution lines 12 are installed aboveground to maintain water service to residential and commercial customers. Access to the water main or pipe 10 requires excavation and removal of sections of the pipe, typically 5 feet long. The distance between access points 14 varies greatly in different water main layouts. Typically, they would be several hundred feet apart, the normal distances between 400 to 800 feet. Sections of pipe which are not being cleaned and lined are closed off by existing line valves (if available) or alternatively by temporary plugs 16. The section of pipe which is to be lined then has the tuberculation 18 removed by passing a pipe cleaner assembly (not shown) through the pipeline until all of the tubercular deposits have been removed. The cleaner is either mechanically winched through the pipe section or is propelled by water flow. Water passes through the cleaner to flush solid debris ahead of the unit. The cleaning process may also include pulling a tight fitting circular squeegee or rubber swab (similar to squeegee 19 in FIG. 2) through the section of pipe to remove standing water and remaining loose solids. They may be pulled through t e pipe separately, and they may be pulled through the pipe immediately before the lining machine. In any event, the essentially bare pipe is then ready for cement mortar application.
In the prior art illustrated in FIG. 1 a relatively small diameter pipeline is shown, that is, one having a diameter of less than 24 inches. In pipes having a larger diameter, access is typically through excavated points, and manually operated equipment is used within the water main, such equipment being illustrated in U.S. Pat. Nos. 1,988,329 and 2,168,917. For cleaning relatively small diameter pipe, a remotely controlled pipe-lining apparatus such as the type shown in U.S. Pat. No. 2,704,873 is utilized. In this form of apparatus dry mortar formed of Portland cement 20 and sand 21 is mixed with water in a mixer 22 to strict specifications and is then pumped by a pump 23 to a lining machine 24 through a hose 26 mounted upon a hose reel 28. The lining machine, which is also referred to as a centrifugal applicator, is positioned at the end of the section of the pipe remote from the winch 30 prior to being winched through the pipeline. The centrifugal applicator may be followed by a flexible cone-shaped drag trowel 32, such as that shown in U.S. Pat. No. 4,184,830. The apparatus is then winched through the line 10 by operation of the winch 30 and winch line 34, the apparatus throwing a wet cement mortar 36 onto the cleaned surface of the water main 10. The foregoing method and apparatus is well known in the art.
In the application of the lining processes referred to, it has been necessary to interrupt individual consumer services as a result of the removal of a pipeline from services. Standard practice has been to provide aboveground temporary piping 12. This has resulted in direct and indirect expenses and delays which are reflected in substantially higher prices than might otherwise be justified.
A properly applied lining may last perhaps 100 years, while an unlined pipeline must be cleaned perhaps every six months. In order to avoid the cost of cement mortar lining in place and to reduce the out-of-service time, many utilities are nevertheless continuing the practice of repeat cleaning because initial cost is less and because its speed of completion allows for a tolerable interruption of customer service so that temporary aboveground service piping is not required. While this procedure is initially cost and time effective, when one considers that it must be repeated again and again, it loses its cost and time effectiveness. In order that the interruption of service during lining may be tolerable, it is desirable to be able to re-introduce water into a water main within about two hours of cement mortar lining so that the rehabilitation of a length of main may be completed within a working day. This can be done if the cement mortar can be provided with a crust which is hardened sufficiently in 1 hour after lining to receive water without erosion damage.
Various proposals have been made for reducing the out-of-service time when a cement lining is applied. Seal coats, such as disclosed in U.S. Pat. No. 4,252,763, have been used in factory cement mortar-lined cast and ductile iron pipes for retaining residual moisture for curing and for helping prevent excessive cracking due to moisture loss. They also help to conceal imperfections and cracks in the cement mortar lining. The seal coats present barriers between the mortar and water carried in the pipe to retard the rate at which soluble elements (primarily calcium hydroxide), which may adversely affect water quality by imparting a high alkalinity (high pH) to the water for the first few weeks after a new cement mortar lining is placed in service, are dissolved from the surface of a new lining. However, such seal coats do not affect the chemistry of hydration of the cement in the mortar and therefore do not accelerate its set time.
As pointed out in U.S. Pat. No. 4,252,763, the application of an asphaltum curing control liquid, as in earlier patents such as U.S. Pat. Nos. 1,988,329 and 2,168,917, was not satisfactory because the freshly applied wet mortar does not strongly adhere to the pipe surface, and, if the lining is disturbed to any substantial extent before it has taken its initial set and has begun to acquire structural strength of its own, it is likely to result in progressive erosion wherein the entire mortar lining pulls away and falls to the bottom of the pipe. In order to overcome these problems, U.S. Pat. No. 4,252,763 proposed to atomize the liquid so that a fine mist of the "curing compound" was applied to the surface of the wet mortar. It is difficult to effect adherence of this seal coat to a fresh lining since it tends to flake off into the water. Moreover, it does not accelerate its set time or provide resistance to scouring by early introduction of flowing water.
Another proposal which has been suggested is to use accelerators within the mortar mix. The use of accelerators has its limitations because of the complexity of the lining procedure which, after the mixing of all the ingredients of the mortar in a mixer, requires the mixture to be kept a certain amount of time in a pump hopper. The cement mortar is then pumped through a hose, which is typically 1 to 2 inches in diameter, for distances of from 400 to 800 feet to a centrifugal machine operated well beyond reach inside the water main. The potential for disaster in the event of unforeseen delays or of unpredictable set time would make even the conservative use of accelerators risky. These accelerators are subject to wide variations in time required for set and are therefore not dependable in field circumstances where wide temperature and material variations are probable. Second, even though the mortar quickly attains initial set through the use of accelerators, it is still vulnerable to surface scouring by a flow of water until it becomes harder. This would require an unacceptable delay in returning a water main to service.
In addition to the above problems of achieving a rapid set time so that out-of-service time can be reduced, there remains the problem of the previously discussed release of high pH constituents into the water of a water main when it is first returned to service thereby reducing water quality.
U.S. Pat. Nos. 2,917,778 to Lyon Jr. et al, 4,362,679 to Malinowski, 4,093,690 to Murray, and 4,746,481 to Schmidt disclose processes for curing various manufactured articles or, in the case of the casting process of Malinowski, floors, walls, and the like on a building site, by the use of carbon dioxide in molds, presses, and the like. The Lyon Jr. et al process involves the closed vessel curing of concrete lined vessels such as water heater shells, and the patent states, at Col. 2, lines 25 to 28, that it is sufficient that the carbon dioxide in the tank be enough to neutralize the layer of calcium hydroxide which is on the surface of the concrete. The time period discussed is undesirably on the order of several days (in Example I, it was 28 days).
While Malinowski, Murray, and Schmidt discuss short curing times ("5 to 30 minutes" in the Schmidt press, and "a few minutes" in the Murray chamber), they do not teach or suggest stopping a reaction of carbon dioxide with deposited cement mortar before a crust of calcium carbonate has been formed through the entire thickness of the cement mortar deposit, and such a crust has a low pH.
Other patents which may be of interest include Japanese patent document 114,823, and U.S. Pat. Nos. 825,088; 898,703; 1,932,150; 2,363,226; 3,249,665; 3,358,342; 4,252,763; 4,350,567; 4,427,610; 4,436,498; 4,772,439; and 5,051,217.
By providing a crust of calcium carbonate, high pH constituents may be prevented from release into the water so that the water quality is not reduced, as previously discussed. However, in using carbon dioxide gas to form the crust, it is important that the water main be returned to service quickly and that the layer of mortar adjacent the pipe maintain a high pH for corrosion-resistance at the pipe interface. It is also desirable that the crust be formed dependably and efficiently.